1. Field of the Invention
The present invention is related to an image signal compressing device for compressing a color still image in accordance with a JPEG (Joint Photographic Expert Group) algorithm. More particularly, the present invention is related to an improvement in the speed at which a discrete cosine transformation (DCT) in a compression process is performed.
2. Description of the Related Art
A standard algorithm for the encoding of high resolution images and the transfer of that information through telecommunication transmission channels has been recommended by the JPEG. In order to enable a large-scale data compression, the baseline process of the JPEG algorithm breaks down the original image data into components on a spatial frequency axis using a two-dimensional DCT. Thereafter, data expressed on the spatial frequency axis is quantized by using a quantization table. The quantized data is then encoded using a Huffman table.
In the two-dimensional DCT and in the quantization process, multiplications, additions and subtractions are carried out many times, respectively. Multiplication is a more complicated process than the addition and the subtraction. Therefore, the larger the number of times a multiplication is carried out, the longer the calculation time in the two-dimensional DC and the quantization. The conventional method in which the number of multiplications is reduced, thus increasing the calculation speed, was proposed in "High Speed Calculation Algorithm of Orthogonal Transformation" (Digital Signal Process Handbook, Ohm Company Limited, issued in 1993). This method however, was provided for the processing of one-dimensional DCT. Therefore, if the method is carried out twice, the method can be applied to a two-dimensional DCT, but there is an upward limit on the calculation speed.